The storage of hydrocarbons in naturally occurring conventionally mined, or solution-mined subterranean cavities is well known. Generally, the subterranean cavity is entirely filled with hydrocarbons and an immiscible displacing liquid, such as saturated brine. The hydrocarbons and immiscible displacing liquid are housed within the cavity in separate phases. When it is desired to introduce additional hydrocarbons into the cavity, a corresponding volume of brine is simultaneously withdrawn. Conversely, when it is desired to withdraw hydrocarbons from the cavity, it is displaced therefrom with a corresponding volume of brine introduced simultaneously into the cavity. Such on-shore storage systems and methods are generally accepted.
Salt storage caverns have most often been used as the subterranean cavity. A cavern is formed in a salt dome or strata by leaching. It is commonplace to drill into the soluble salt formation and produce brine by solution mining, well known in the art. Dual fluid passages are provided into the cavern so that fluid handling means at the surface provide a capability for brine to be pumped into and out of the lower area of the cavern and hydrocarbons then can be taken from the upper area. Hydrocarbons are added to storage by pumping the hydrocarbons into the cavern under sufficient pressure to displace the brine therein back to the surface. The displaced brine is maintained at ground level in a brine pit or reservoir, and then is returned to the salt cavern to replenish the volume of brine as hydrocarbons are retrieved. Precautions are taken to assure that the brine in and out of the cavern is always salt-saturated to avoid enlarging the brine cavern.
The development of large oil reserves offshore and the off-loading of ultra-large crude carriers or supertankers at offshore terminals have introduced many problems relative to the transportation and storage of the hydrocarbons. Typically, the hydrocarbons being produced or off-loaded are transported to a shore terminal by pipeline or lightering (barge or other small vessel). The pipeline extending from the offshore terminal to shore often is many miles long and must have sufficient capacity to handle the rate of flow of crude oil being off-loaded from the supertanker. Supertankers typically pump up to 100,000 barrels of crude per hour form their holds to an offshore platform where the crude oil enters the pipeline extending to shore. For the pipeline to have sufficient capacity to off-load 100,000 of crude per hour from the supertanker, a plurality of large pipelines sometimes 52 inches in diameter or more may be needed to have sufficient capacity to off-load the supertanker at that flow rate. These large pipelines are expensive and often have a cost in the millions to billions of dollars. Thus, the pipelines to shore account for most of the cost of constructing offshore terminals for supertankers. Obviously, this cost is directly dependent upon the distance of the offshore terminal from shore.
Technical feasibility studies have been made of utilizing an underground storage of crude oil for an offshore terminal. See "Offshore-Terminal with Underground Storage at Sea" by P. C. Relotius, H. Lorenzen, and H. Kaundinya, published in the Oil Gas European Magazine, Vol. 11 at pp. 39-45, Nov. 1977. Although in the past subsea tanks have been considered for the storage of hydrocarbons, such tanks tend to collapse under the water pressure and have severe maintenance problems. Subsurface caverns at sea have the problem of how to store the necessary displacement liquid. The present invention overcomes these deficiencies of the prior art.
Natural subsea brine ponds occur in many parts of the world and especially in the Red Sea and the Gulf of Mexico. Such subsea brine ponds have been studied as sources of elements like iodine and bromine and as sources for dissolved natural gas. The present invention either makes use of an existing subsea brine pond or creates a new subsea brine (or other liquid) pond to provide storage for displacement liquid for use in drawing down subsurface hydrocarbon storage caverns.